1. Field of the Invention
This invention relates to a toner for developing electrostatic images, and more particularly, to color toners such as magenta, cyan, and yellow toners for developing electrostatic images.
2. Description of the Prior Art
Heretofore, various electrophotographic methods have been known. For example, U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 23910/1967 and 24748/1968 disclose electrophotographic methods. In general, these electrophotographic methods utilize photoconductive materials as the photosensitive material and comprise charging, imagewise exposing to form electric latent images on a photosensitive member, developing the latent images with a toner, and if desired, transferring the developed images to a web such as paper and fixed by, for example, heating, pressing or applying a solvent vapor. When multi-color images are desired, the original light image is projected to a photosensitive member through a color separation filter, and the above mentioned procedure is repeated by using various filters and the corresponding color toners such as yellow, magenta and cyan toners, and the toners are overlaid subsequently to produce a color image.
There are known toners composed of coloring materials such as carbon black, metal-containing dyes, pigments and the like dispersed in resin binders such as polystyrene and ground to fine powders of 1-50 microns in size. These toners are usually mixed with carriers such as glass beads, iron powders, fur and the like, for developing electric latent images. These toners are requested to have particular chemical and physical properties desirable for electrophotography and other practical purposes.
However, conventional toners often have the following drawbacks. Most toners capable of being easily melted by heating are apt to agglomerate during storage and upon handling. Most toners are adversely affected by changes in ambient humidity and thereby the triboelectric characteristics and fluidity characteristics are degraded. When conventional toners are used, the toner, a carrier and the surface of a photosensitive plate are all deteriorated by collision between the toner particles and the carrier and contact of the toner particles with the surface of the photosensitive plate as the result of repeated and continuous use of the toner. Therefore, the resulting image density is not constant, but changes, and the background density increases to deteriorate the image quality.
When it is tried to increase the image density by increasing the toner amount to be attached to the surface of a photosensitive plate, the background density also increases and fog is formed in most cases of conventional toners.
In case of color electrophotography according to a multi-color overlaying process, the color toner should have the following particular characteristics as well as excellent physical and chemical properties overcoming the above mentioned drawbacks.
(1) The toners should have a high transparency because different color toners are to be overlaid.
(2) The toners should be melt-miscible.
(3) Spectral reflection characteristic should be excellent so as to reproduce the original with high fidelity.
In addition, it is very difficult to use a charge control agent for imparting a desirable polarity of triboelectric charge to a toner in the case of a color electrophotographic toner. Therefore, it is necessary that a toner can be selectively charged negatively or positively as to a certain carrier to be used together by selecting appropriately only a colorant and a resin binder. In general, color electrophotographic toners must satisfy various conditions and therefore, it is not easy to satisfy simultaneously such various conditions as well as conditions to obtain a desirable polarity of the toner by selecting only a combination of a colorant and a resin binder. In view of the foregoing, the combination of the colorant and the resin binder is very important and the desirable combination is not easily anticipated by prior art.